Currently, a semiconductor laser using a nitride semiconductor has an increased demand on utilization of an optical disk system which can record and reproduce information at a large capacity and a high density. For this reason, a semiconductor laser device using a nitride semiconductor has been intensively studied. In addition, it is thought that a semiconductor laser device and a light emitting device using a nitride semiconductor can be oscillated at a wide region from ultraviolet to red color. Therefore, it is applied not only to a light source for the aforementioned optical disk system but also to a light source for a laser printer and an optical network. The present applicant have already announced a laser which can continuously oscillate for longer than ten thousands hours under the conditions of 405 nm, room temperature and 5 mW.
In addition, a laser device, a light emitting device and a light receiving device using a nitride semiconductor have a structure in which an active layer may be formed by using a nitride semiconductor containing In and, therefore, formation of a more excellent active region in an active layer becomes important for improving the device properties.
In addition, in a nitride semiconductor device, particularly, in a laser device and a light emitting device, light emitting and oscillation at a wavelength of 380 nm or shorter are highly required. In the aforementioned optical disk system, a recording density is improved by a shorter wavelength and, in a light emitting device, a nitride semiconductor device becomes important as a light source for exciting a fluorescent body. Also in other applications, many uses can be realized by a light source of further shorter wavelength.
In order to obtain light emitting at a short wavelength in a nitride semiconductor laser device or light emitting device, an emitting wavelength can be varied by changing an In crystal mixing ratio in a nitride semiconductor containing In in an active layer or a light emitting layer and more specifically, an emitting wavelength can be shortened by decreasing an In crystal mixing ratio. In addition, when an active layer has a structure in which said active layer is sandwiched between an upper cladding layer and a lower cladding layer in an end emitting device or a laser device, by rendering a refractive index of both cladding layers small and rendering a refractive index of inside of a waveguide between an upper cladding layer and a lower cladding layer high, the light is effectively confined in a waveguide, which results in contribution to decrease in a threshold current density in a laser device.
However, as a wavelength grows shorter, it becomes difficult to use a quantum well structure of InGaN or InGaN/InGaN which have previously been used as a light emitting layer and, at a wavelength of not greater than 365 nm corresponding to a band gap for GaN, it becomes difficult to use InGaN as a light emitting layer. In addition, when a wavelength becomes shorter, the loss occurs due to light absorption in a guiding layer in a waveguide, resulting in an enhanced threshold current. Further, also in light confinement by an upper cladding layer and a lower cladding layer, since the use of GaN maintains a difference in a refractive index for loss due to light absorption and light confinement in a waveguide, it is necessary to use a nitride semiconductor having a great Al ratio and, thus, a problem of the crystallizing property becomes more important.
In addition, as an attempt to make a wavelength of such the nitride semiconductor device shorter, a quantum well structure of AlGaN/AlGaN is used and, however, there is a tendency that sufficient output can not be obtained as compared with the conventional InGaN system.
In addition, in the case where a nitride semiconductor containing Al such as AlGaN is used in an device, a difference in the thermal expansion coefficient and the elasticity are greatly different as compared with other nitride semiconductors containing no Al and, thus, when a nitride semiconductor containing Al is used, crack is easily produced, production of crack destroys an device unlike the other crystallizing properties and, therefore, if crack is not prevented from occurring, an device does not serve as a nitride semiconductor device. For this reason, in a light emitting device and a laser device using the aforementioned active layer having an emitting wavelength of 380 nm or shorter, since a nitride semiconductor containing Al can make the band gap energy greater in a nitride semiconductor, it is used in an active layer, as well as a carrier confining layer, a light guiding layer and a light confining layer having the greater band gap energy than that of the active layer. That is, in the aforementioned light emitting device of a shorter wavelength area, a nitride semiconductor containing Al has a multi-layered structure. On the other hand, the aforementioned problem of production of crack becomes serious and, therefore, there is a tendency that shorter wavelength and prevention of crack production has the contradictory relationship, and this becomes a serious barrier to more shorter wavelength in a light emitting device of a nitride semiconductor. Further, since GaN has an absorption end for light at 360 nm and has a high absorption coefficient even at a region of a longer wavelength than that of the end by around 10 nm, it becomes difficult to use GaN in a light emitting device and a laser device in the aforementioned shorter wavelength area of 380 nm or shorter.
In addition, since an active layer in a light emitting device or a laser device has the emitting efficacy and the internal quantum efficacy depending greatly upon the crystallizing property thereof as described above, the crystallizing property of an electrically conductive type of layer arranged below an active layer becomes an extremely important factor for improving the properties of an device. Usually, a nitride semiconductor light emitting device has a structure in which a n-type layer, an active layer and a p-type layer are laminated in this order and, in this case, it is necessary to make the crystallizing property of a n-type layer better. On the other hand, as described above, there is a tendency that the crystallizing property of a nitride semiconductor containing Al is greatly deteriorated as compared with other nitride semiconductors containing no Al and, previously, for the purpose of avoiding such the problem, a nitride semiconductor layer containing In is used as a substrate layer for a nitride semiconductor containing Al to alleviate occurrence of the internal stress due to a difference in the thermal expansion coefficient, a nitride semiconductor containing no Al such as Ga is provided adjacent to a nitride semiconductor layer containing Al to realize recovery of the crystallizing property and alleviation of the internal stress, whereby, it allows an device such as a laser device having a structure in which a nitride semiconductor layer containing Al is provided therein, to be practically exerted. However, in the aforementioned light emitting device and laser device having a shorter wavelength, a nitride semiconductor containing no Al becomes a light-absorbing layer and use of it in an device structure is not preferable and, for this reason, most of device structures use a nitride semiconductor layer containing Al. Thus, a light emitting device and a laser device having the practical threshold Vf and emitting efficacy can not be obtained due to the aforementioned crystallizing property and occurrence of crack and, in particular, in a laser device which uses much nitride semiconductors containing Al and having a greater Al crystal mixing ratio in a light-guiding layer or a cladding layer for light confinement, a laser device which can be continuously oscillated at room temperature can not be obtained.